1. Field of the Invention
The invention relates in general to electrical discharge machining (EDM) and, in particular, to a method for controlling the precision forming of deep cuts such as, for example, blind holes, and other openings using EDM equipment.
2. Description of the Prior Art
EDM techniques had been used extensively for forming, among other things, precise cuts such as, for example, holes or openings, particularly blind holes, in extremely hard, electrically conductive materials, with the limitation that the maximum possible depth of such blind holes generally was believed to be less than approximately 0.060 inches. This limitation occurred because, as the machining surface of the bottom of the hole was machined by the cutting pulse spark of the EDM electrode, debris accumulated in the spark gap, induced arcing, and prevented further significant machining of the machining surface Various expedients, including hydraulic flushing of the spark gap, had been proposed. Vigorous hydraulic flushing can deflect small electrodes so as to impair the accuracy of the operation. The problem of machining small, deep, blind holes, while maintaining a desired size, shape and finish for the hole, had generally proven to be unsolvable.
The debris which is inevitably formed by EDM operations is generally composed largely of very small particles of the material which is machined from the workpiece, and carbon formed by thermal degradation of the cutting fluid. The debris can, and often does, build up in the spark gap, and may even bridge between the electrode and the workpiece so as to cause arcing. This causes at least the working gap voltage, and the characteristics of the radio frequency emitted by the machining process to change. It also prevents any further significant machining of the workpiece, because all or most of the energy is going into reworking debris. Unless it is removed the debris will eventually make further machining impossible. Previous expedients have employed adaptive controls in an attempt to overcome the problem of energy loss or arching in the debris. U.S. Pat. Nos. 4,357,516, 3,056,065, and 3,943,321 generally disclose the application of a succession of individually discrete pulse trains to the spark gap separated by a long off-time in response to an excessive accumulation of debris in the spark gap. U.S. Pat. No. 4,338,504 discloses monitoring radio frequency emissions of the cutting pulse spark, and in response to a change in radio frequency emissions, the cutting pulse spark is interrupted. The radio frequency emissions change is an indication of excessive accumulation of debris in the spark gap, particularly carbonaceous debris. Hydraulic flushing of the spark gap by the dielectric fluid was previously induced by physically moving or pulsing the electrode in and out of the spark gap as a way of dealing with debris accumulation. Such attempts to overcome the problem of arcing and energy loss in the debris had generally become completely ineffective in, for example, deep, small, blind holes at depths of somewhere between about 0.030 and 0.060 inches, and with large electrodes. The present invention is particularly applicable to accurately machining small, deep, blind holes where the ratio of the depth of the hole to the smallest lateral dimension of the hole is greater than approximately 1:1.
The limitation on the depth at which previous EDM processes could machine small blind holes severely restricted the utility of such EDM operations. It should be noted that all fully surrounded holes are blind holes while they are being formed, unless an opening has been previously made in the workpiece. Likewise, all through holes are blind holes while they are being formed, until they break through the workpiece. Thus, the previous inability to form small, deep, blind holes had limited the depth of all small diameter holes. Problems had also been encountered with large holes particularly where there is a considerable distance between the center and the edges of the electrode. The concentration of debris is apparently greater at the approximate center of a large electrode than around its edges. Physically, such debris has further to travel to escape from the spark gap than that which is generated near the edges of the electrode.
These and other difficulties of the prior art have been overcome according to the present invention. Those concerned with these problems recognize the need for an effective and reliable way to accurately machine small, deep, blind holes of nearly unlimited depth by the EDN process.